The present invention relates to a technique for monitoring film thickness including a method of controlling etch and deposition processes. One specific embodiment of the present invention relates to a method of controlling and detecting the end point in a plasma etch process by using the plasma emission as a broadband light source and measuring the radiation reflected off the wafer being etched with a spectrometer. The present invention may be used to control a variety of plasma and non-plasma processes and is particularly useful in controlling plasma etch processes used in the manufacture of integrated circuits.
Etching one or more layers of a semiconductor substrate in a plasma etch environment is one of the most common steps in the manufacture of integrated circuits. Typical plasma etch processes in semiconductor manufacturing are controlled and stopped either as a timed process or by relying on an end point detection (detection of the time at which the film being etched is completely removed) technique that monitors the optical emission lines of certain species in the plasma.
As new generations of integrated circuits employ smaller feature sizes than were contemplated in previous generations, greater demands are placed on the fabrication process. These demands include being able to precisely control the timing of plasma etch processes. For example, one common process used to etch a polysilicon gate structure includes three separate steps. The first step removes a top oxide layer that may have formed over the polysilicon, the second step etches the bulk of the polysilicon at a relatively high etch rate to increase wafer throughput and the third step switches to a slower, but more selective (relative to the underlying oxide layer) final etch.
To preserve the integrity of the underlying gate oxide layer (which may be less than 25 xc3x85 thick) in such an etch, it is necessary to have an endpoint prediction technique that triggers a process switch to obtain high selectivity to the gate oxide near the end of the plasma etch. Because of variations from one etching step to another, a timed etch may result in under- or over-etching of the polysilicon layer. Similarly, employing an endpoint detection technique that monitors optical line emissions relies on exposing the underlayer (i.e., the gate oxide in this example) to the plasma etch chemistry. Such exposure can degrade the integrity of the gate oxide layer. Thus, to avoid underlayer exposure to the plasma environment, an endpoint prediction technique that directly monitors the wafer state is required.
One process that has been developed to meet this requirement is an interferometric endpoint (IEP) detection system. The IEP system uses a mercury (Hg) lamp that generates a strong Hg atomic line emission that is coupled to a bifurcated optical fiber. A collimating lens and folding mirror set at the output end of the fiber illuminates a spot diameter of approximately 1 cm in diameter on the wafer through a sapphire window mounted on the top of the dome. The light reflected from the wafer is coupled back to the second arm of the bifurcated fiber and measured by a single wavelength optical detector, such as a photomultiplier or photodiode. The single wavelength optical signal shows optical fringes as the thickness of the top layer film is etched away. For weakly or non-absorbing films fringe counting can be used for etch rate measurements and etch-to-depth prediction. However, some films have higher absorption and the fringes will be observed only below a certain film thickness. Currently, the single wavelength detection limits the IEP system to the existing Hg lines, usually the useful lines are 254, 313, 365, 405 and 435 nm. As device features shrink it is more advantageous to use shorter wavelengths (UV) lines that can better resolve thinner films. Moreover, film properties such as spectral dependent absorption may require the use of optical lines other than those available by Hg lamp.
Another technique that has been developed to detect the endpoint of plasma etch processes uses a CCD camera with appropriate sensors, filters, optics and software to monitor the reflection of a single wavelength of radiation from numerous spots on the wafer. This technique can be used to determine the etch rate of an etch process by examining and comparing the timing of two or more fringes. The technique cannot directly determine film thickness and instead can be used to estimate thickness based on the thickness of the film as deposited and the measured etch rate.
In a different technique that has been employed to monitor the growth of epitaxial films, multiple wavelengths of radiation reflected off the wafer are measured with a spectrograph. The technique then creates a model of film growth based on optical constants such as the refractive index of the film to account for film thickness, growth rates, etc. This technique requires that separate models, which can be quite extensive, be developed for controlling different processes and even variations in final film thickness, temperature conditions, etc. of a same process. The models become even more complicated when taking signal variations due to wafer patterns into account.
Accordingly, new techniques to in situ monitor film thickness in etching and deposition processes are desirable.
The present invention provides an improved technique to monitor film thickness including an improved plasma etch monitoring and endpoint prediction technique. One embodiment of the invention uses the plasma emission of the etch process as a broadband light source and measures the reflectance off the wafer with a spectrometer. A plurality of the measured wavelengths are then compared using pattern recognition techniques to previous measurements taken during an earlier, preferably pre-production calibration run to control the plasma etch process. The previous measurements act as xe2x80x9cfingerprintsxe2x80x9d of a particular process for a particular integrated circuit design that can be subsequently compared to data measured from the same process performed on a subsequent wafer used in the manufacture of the same integrated circuit design. The fingerprints can be taken at various times of the etch process and used as check points to time the process by identifying when the thickness of the production film matches the thickness of the calibration process film at a previously identified point. Once the xe2x80x9cmatchingxe2x80x9d thickness is found, actions such as altering the process chemistry or stopping the process altogether when its endpoint is reached can be taken. Other embodiments monitor and control the thickness of a deposited film, and some embodiments measure the reflectance of radiation off the surface of the wafer generated by a light source other than the plasma emissions, e.g., a mercury, deuterium or xenon lamp.
One particular embodiment of the method of the present invention uses principal component analysis (PCA) techniques to perform the pattern recognition comparison. PCA basically transforms the input variables to a set of orthogonal vectors, referred to as principal components (PCs), which is linear combination of the original variables. The technique capitalizes on the fact that very often many parameters are correlated. The PC vectors (eigenvectors) are ordered by their eigenvalues. Typically two PC vectors capture more than 90% of the variations in the signals, therefore, minimizing the dimensionality of the system. Another embodiment of the invention uses programmed neural net pattern recognition techniques.
These and other embodiments of the present invention, as well as its advantages and features, are described in more detail in conjunction with the text below and attached figures.